There is known in the art to measure the level of the surface of a medium stored in a container by means of a radar level gauge. Such devices are disclosed in a number of documents. As an example of such a document, it will here be referred to U.S. Pat. No. 4,665,403.
For reasons of security, in a container, in almost all cases, a cargo which requires a closed gauging device also requires both a high level alarm and an overfill control. A cargo tank containing such a product therefore requires three sensors:
1. Level gauging sensor for measuring at all levels.
2. High level alarm.
3. Overfill control.
It should be mentioned here that an overfill control system comprises an overfill alarm system. According to security rules the three listed sensors must function independently of each other. A fault of one sensor must not render either of the other sensors inoperative. International patent publication WO 01/02818 discloses a level gauging system based upon independent radar measuring channels for determining the level of a product in a container. In said publication the three channels in the gauging system use a common antenna. One use of such a system is to use one of each separate channel for the purpose of achieving a level gauge, a high level alarm and an overfill control, each in a separate channel. If it would be possible, at least from cost aspects, a more simple system would be preferred. The invention disclosed in this description shows a solution for arriving at a level gauging system for fulfilling the above mentioned three level gauging functions in a simple and cost reduced way.
Not yet published patent document SE 0200080-0 shows an example of more than one level gauge coupled in parallel to an evaluation unit for determining the best value of, for example, a level value from said level gauges, but there is in said document nothing disclosed about any means for reducing the number of independent level gauges.